Quadrature Phase Shift Keying.



QPSK is a digital frequency modulation technique used for sending data over coaxial cable networks. Since it's both easy to implement and fairly resistant to noise, QPSK is used primarily for sending data from the cable subscriber upstream to the Internet. 



See also: 64QAM, BPSK, DQPSK, CDMA, S-CDMA, VSB



(Thanks to cnet.com)